Vacuum electron devices that generate electromagnetic power in the gigahertz (GHz=109 Hz) to terahertz (THz=1012 Hz) frequency range often rely on very small metal structures, known as “interaction circuits,” that support electromagnetic fields which interact with a beam of electrons in vacuum. Typically, the electrons are guided substantially by an external magnetic field through the interaction circuit(s) without physically touching the walls of the circuit by means of a tunnel, or “beam tunnel,” bored through the interaction circuit. The interaction circuit is often substantially metal, such as copper for low microwave loss and high thermal conductivity, or a metal-coated material such as silicon or diamond. At high electromagnetic frequencies, such as above 100 GHz, these beam tunnels become very small, approximately 0.002 inches to 0.050 inches in diameter for a round beam tunnel, and typically 0.1 inch to 5 inches in length.
Traditional methods for forming the beam tunnels through the interaction circuits include (a) drilling round holes, (b) ablating holes by sinker electrical discharge machining (SEDM), (c) forming the beam tunnel from two halves of the cross section by Wire EDM (WEDM) and then bonding the two halves together by brazing, mechanical fixturing, diffusion bonding or other means, (d) laser drilling the holes, (e) ablating holes by water jet, (f) casting the metal into a mold, (g) multi-layer lithography, electroforming and molding (LIGA) processes. However, these methods are all limited in their ability to reliably and precisely form very small holes of arbitrary shape for long lengths. Specific limitations of each of these methods are discussed below.
In the (a) drilling round holes methods, drilling a beam tunnel typically limits the cross sectional shape to round, and has a limited depth that the bore can be drilled to accurately. Furthermore, during drilling, there is a tendency to walk off-center, and the method relies on extremely fragile micro-drill bits. In the (b) SEDM method, as practiced in industry, there is typically a very limited beam tunnel length that can be fabricated (i.e., approximately 0.3 inches in length with a 0.005 inch diameter hole (aspect ratio 60:1) at the time of this application). Additionally, SEDM also tends to form a conical rather than cylindrical beam tunnel. The (c) WEDM method typically requires either a pilot hole to be drilled, which can be limited by the drilling techniques described in (a), or the method requires that two halves be bonded or brazed together. In brazing, stresses due to cyclical heating typically tend to separate the two halves where they were joined, which can reduce reliability. The (d) laser ablation method is typically limited in the depth of the cut.
The (e) water-jetting method typically produces a cone-shaped hole rather than a cylindrical one due to ballistic scattering. In the (f) casting method, molten metal around a mold requires the forms to be subjected to high temperatures, which can introduce issues of thermal expansion, can complicate the removal of the molds, and typically has a tendency to leave gaps and voids in the casting. In the case of some pure metals such as copper, cast melting tends to form large crystal grains, which are typically unable to fill the smallest features properly. Finally, (g) conventional multi-layer LIGA techniques require a minimum of three layers, each requiring a separate exposure step, electroforming step and polishing step, and these techniques are only capable of square or rectangular beam tunnel shapes or approximations to true cylinders.
Accordingly, there remains a need for an alternative method of forming beam tunnels in a metal structure. Preferably, this new method would allow for forming any number of very small, precise high-aspect-ratio hollow bores, or beam tunnels, of arbitrary cross sectional shape and arbitrarily long length in metals during a single photolithographic process along with the electromagnetic interaction circuit. Therefore, the method could circumvent many of the shortcomings of the prior methods along with providing a more efficient process.